Method and apparatus for acquiring a frequency without a reference clock

ABSTRACT

A clock and data recovery system acquires a clock embedded in an input data stream by detecting the occurrence of transitions in the input data stream falling into a predetermined phase zone of a sample clock used to sample the input data stream. A control circuit counts how many evaluation intervals have at least one transition in the predetermined phase zone. The control circuit determines if lock is achieved according to the count. If it is determined that lock is not achieved, an output of a variable oscillator circuit used in the clock recovery operation is adjusted until the number of evaluation intervals having one or more transitions in the predetermined phase zone is below a level indicating lock.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to data communication and moreparticularly to acquisition of a clock signal associated with the datacommunication.

2. Description of the Related Art

Communication systems frequently transmit data in which the clock isembedded in the data stream rather than sent as a separate signal. Whenthe data stream is received, a clock and data recovery circuit recoversthe embedded clock and retimes the received data to the recovered clock.Traditionally, a phase-locked loop (PLL) has been used to perform theclock recovery operation. FIG. 1 shows a block diagram of a traditionalPLL configured for a clock and data recovery application. Thephase-locked loop 100 includes a phase detector 102, which receives theinput data signal conveyed on node 104 and also receives the VCO outputclock signal conveyed on node 106 from the voltage controlled oscillator(VCO) 108. The phase detector 102 generates an error signal 110, whichis a function of the phase difference between the input data signal andthe VCO output clock signal. The phase detector 102 may also includeadditional circuitry to generate the reconstructed data on output node114.

In order to help the VCO acquire the frequency of the input data stream,it has been common to use a reference clock to center the VCO outputfrequency for a nominal output that approximates the frequency of theinput data stream. In a typical application, the VCO will multiply thereference clock by a predetermined (or selectable factor), e.g., 16, toachieve the nominal VCO output. For example, if the multiplicationfactor is 16, for a 2.7 Gbps data rate, the reference clock is 168.75MHz. The requirement for a reference clock (generally differential) addsboth cost and design complexity to the system in which a clock and datarecovery circuit resides. The clock has to be supplied by a relativelyhigh cost crystal oscillator component and distributed to the clock anddata recovery circuit using design practices appropriate for high speedclock signals.

In addition to using the reference clock to center the nominal output ofthe PLL, the reference clock is also used to determine whether lock hasbeen achieved. Typical lock-detect circuitry compares the referenceclock to a divided down version of the recovered clock, and if thedifference between the two clocks is sufficiently high, the PLL isdetermined to be out-of-lock.

It would be desirable to be able to provide a communication system thatcan acquire a clock signal embedded in an input data stream withouthaving to use a reference signal. That would both save pins on the clockand data recovery integrated circuit and release the system from thecost and design complexity of having to supply a reference clock.

SUMMARY OF THE INVENTION

Accordingly, a method and apparatus is provided for acquiring afrequency without using a reference clock. In one embodiment, a methodis provided for acquiring a clock embedded in an input data stream, byvarying an output of a variable oscillator until transitions of theinput data stream occurring in a predefined phase zone of a sample clocksampling the input data stream occur at an acceptable rate. Theacceptable rate is determined according to a number of evaluationintervals having one or more transitions occurring in the predefinedphase zone.

In another embodiment, a method for reference-less frequency acquisitionincludes detecting whether transitions of the input data stream fallinto a predetermined portion of a sample clock period of a sample clockutilized in a phase detector to sample the input data stream andevaluating whether a phase-locked loop (PLL) has acquired the timing ofthe input data stream according to the occurrence of transitions of theinput data stream in the predetermined portion of the sample clockperiod.

In another embodiment, the invention provides an integrated circuit forreceiving an input data stream and locking to a timing of the input datastream using a phase-locked loop. The integrated circuit includes aphase zone detect circuit coupled to determine if a transition of inputdata stream occurs in a predetermined phase zone of a sample clock. Theintegrated circuit further includes a counter circuit coupled to thephase zone detect circuit to supply a count indication of a number ofevaluation intervals in which at least one transition in the phase zoneoccurs. A compare circuit compares the count indication and a thresholdvalue and outputs a compare indication indicating if the PLL has lockedto the input data stream. A variable oscillator circuit forms part ofthe PLL. A control circuit adjusts the output of the variable oscillatorcircuit until lock is achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention may be better understood, and its numerousobjects, features, and advantages made apparent to those skilled in theart by referencing the accompanying drawings.

FIG. 1 shows a block diagram of a traditional PLL configured for clockand data recovery operations.

FIG. 2 shows a clock and data recovery architecture that can be modifiedfor use with the present invention.

FIG. 3 shows additional details of the clock and data recovery circuitshown in FIG. 2.

FIG. 4A illustrates an exemplary voltage controlled oscillator (VCO)used in the present invention.

FIG. 4B is a circuit diagram of an illustrative embodiment for one ofthe banks of capacitors utilized in the VCO shown in FIG. 4A.

FIG. 5 depicts a common phase detector.

FIG. 6 shows a timing diagram illustrating operation of the phasedetector shown in FIG. 5.

FIG. 7 illustrates the ideal placement of data transitions within thephase window of the clock used to sample the data.

FIG. 8 illustrates the movement of the transitions clockwise withrespect to the 180 degree point of the sample clock in the phasedetector.

FIG. 9 illustrates the effect of phase offset and increased jitteramplitude on the location of the data transitions.

FIG. 10 shows the movement of the data transitions from quadrant toquadrant as a result of frequency offset.

FIG. 11 illustrates the “forbidden zone” of the sample clock.

FIG. 12A illustrates an augmented phase detector circuit that detectstransitions in the forbidden zone.

FIG. 12B illustrates another embodiment of an augmented phase detectorcircuit that detects transitions in the forbidden zone.

FIG. 13A illustrates data transitions at 180 degrees with respect to thephase of the sample clock signal.

FIG. 13B illustrates the operation of the phase detector shown in FIG.12A when data transitions occur in the forbidden zone.

FIG. 14 illustrates a Bernoulli process.

FIG. 15 illustrates uniform distribution of transitions across the phaserange.

FIG. 16 illustrates an exemplary high level flow diagram of the controlstructure utilized to acquire a frequency.

FIG. 17 illustrates one preferred approach for the stepping algorithmused to vary the capacitor settings.

FIG. 18 illustrates one embodiment of an augmented phase detector thatdetermines when transitions occur in the forbidden zone.

FIG. 19 illustrates one embodiment of an augmented phase detector thatdetermines when transitions occur in the forbidden zone.

FIG. 20 illustrates the implementation of the delay associated with theOC3 clock and the generation of the clocks for OC24, OC 12 and OC3 modeof operations.

FIG. 21 illustrates implementation of the delay associated with OC24 andOC12 clocks.

FIG. 22 illustrates interpolating between two clocks to achieve ashorter delay than can be achieved by utilizing one delay buffer.

FIG. 23 shows a block diagram of a portion of an exemplary clock anddata recovery circuit incorporating the augmented phase detector.

FIG. 24A shows the probability of falsely asserting out-of-lock withvarious bit error rates (BER) and for various bit error counts for 16trials.

FIG. 24B shows the probability lock is falsely declared versus count andwidth of the forbidden zone for 16 trials.

FIG. 25A shows the probability of falsely asserting out-of-lock withvarious bit error rates (BER) and for various bit error counts for 48trials.

FIG. 25B shows the probability lock is falsely declared versus count andwidth of the forbidden zone for 48 trials.

FIG. 26A shows the probability of falsely asserting out-of-lock withvarious bit error rates (BER) and for various bit error counts for 512trials.

FIG. 26B shows the probability lock is falsely declared versus count andwidth of the forbidden zone for 512 trials.

FIG. 27A shows a table that indicates one embodiment for assigningdigital values indicating a bit error rate.

FIG. 27B shows a table that indicates one embodiment for assigningdigital values indicating a bit error rate to achieve greaterresolution.

FIG. 28 shows the probability that the measured count of transitions inthe forbidden zone (the bit error count) is greater than the bit errorrate for 512 trials when the PLL is in-lock.

FIGS. 29-31 illustrate the operation of stepping the capacitance valuesof the VCO while trying to acquire lock.

FIGS. 32-40 illustrate simulation of frequency detection under variousconditions.

FIG. 41 shows a block diagram of a clock and data recovery integratedcircuit incorporating various aspects described herein.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

Referring to FIG. 2, a clock and data recovery architecture 200 that canbe m utilized in conjunction with the present invention is illustrated.As seen in FIG. 2, a phase detector 210, an augmented version of whichis described further herein, is followed by a delta-sigma modulator 212.The phase detector 210 compares the phase of the input data signalconveyed on node 214 to the phase of the recovered clock signal conveyedon node 216, and generates a pulse width modulated error signalwaveform. In this example, the error signal is a current waveformflowing into or out of node 218. The delta-sigma modulator 212 thenconverts the pulse width modulated error signal into a discrete-time anddiscrete-amplitude digital output signal, in this example generating aone-bit digital output on its output node 220.

As shown in greater detail in FIG. 3 in one embodiment, the delta-sigmamodulator 212 includes a modest-sized (e.g., having a typical value of2-3 pF) integrating capacitor 222 connected to node 218, and furtherincludes a digital comparator block 226 which samples the voltage on itsinput node 218 when clocked by a delta-sigma clock received on clocknode 224. The digital output generated on the output node 220 is fedback as a negative current by feedback block 228 into node 218 toprovide the requisite feedback into the integrating capacitor 222 of thedelta-sigma modulator. Operation of such first-order delta-sigmamodulators is well-known to one skilled in the art Further details onthe clock and data recovery circuit shown in FIG. 2 can be found inapplication Ser. No. 60/217,208, filed Jul. 10, 2000, which isincorporated herein by reference in its entirety.

Referring again to FIG. 2, the loop filter for this clock and datarecovery circuit includes a feedforward path 230 formed by a gain block232 and a filter block 234, and further includes a digital integratingpath 240. The output signal of the feedforward path 230, which isconveyed on node 236, and the output signal of the digital integratingpath 240, which is conveyed on node 249, are combined by summer block250 to generate a control signal (V_(g)) on node 252 for the voltagecontrolled oscillator 260.

The digital integrating block 240 includes a decimator 242, anaccumulator 244, a D/A converter 246, and a filter block 248. Thedigital accumulator 244, which includes a multiple-bit register torepresent the cumulative value of the phase error, receives a decimateddigital phase error representation from decimator 242, and increases ordecreases the cumulative value accordingly. For example, if the digitalphase error representation corresponds to a “leading” phaserelationship, the digital accumulator 244 will increase (or alternately,decrease) the cumulative value stored in its output register.Conversely, if the digital phase error representation corresponds to a“lagging” phase relationship, the digital accumulator 244 will decrease(or alternately, increase) its cumulative value stored in its outputregister. While this and other block diagrams are described using theterminology of a single node connecting the blocks, it should beappreciated that, when required by the context in the variousembodiments, such a “node” may actually represent a pair of nodes forconveying a differential signal, or may represent multiple separatewires (e.g., a bus) for carrying several related signals.

As described above, the multiple-bit output register of the digitalaccumulator 244 holds a digital representation of an integrated value ofthe phase error. This digital value is preferably communicated on anN-bit wide output bus 245 to the D/A converter 246 which converts thedigital representation back into an analog signal. Filter block 248provides a smoothing function to the reconstructed analog signal. Theoutput of the filter block 248 is then conveyed on node 249 to thesummer block 250.

As stated above, because a phase detector having a digital output signalis used, the requirement for a separate analog-to-digital converter,which exists only in the digital integrating block, is eliminated. Thisallows a low offset to be achieved in the phase error through thefeedforward path because both the feedforward path and the integratingpath receive the same digital signal.

An exemplary voltage controlled oscillator (VCO) 260 is shown in FIG.4A. In one embodiment, the voltage controlled oscillator is implementedas a tank circuit (an LC oscillator) in which the frequency ofoscillation depends on the inductance (L) 402 and capacitance (C) 404.In the illustrated embodiment, the output of the VCO 260 is adjusted byvarying the capacitance 404, which includes separately controllablecapacitor circuits. The summer circuit supplies the control voltage(V_(g)) on node 252 to adjust the variable capacitance 410 circuit. Thevoltage V_(g) controlling the capacitance setting is an analog voltageand thus the capacitance is indicated as being an analog capacitance. Inaddition to the variable capacitance circuit 410, variable capacitancecircuit 412 accounts for drift and other long term variations that canaffect operation of the VCO. The fixed capacitance 414 is used toprovide a capacitance that approximates the desired value and canrepresent parasitic capacitance in the circuit. Variable capacitancecircuit 416 provides a digitally controlled variable capacitance thatcan be used, as described further herein, to acquire the frequency ofthe input data stream without the use of a reference clock. Variablecapacitance 416 in one preferred embodiment includes variable capacitorsconfigured to allow coarse grained, medium grained and fine grainedadjustment of capacitance settings as described further herein. Anexemplary variable capacitance circuit is shown in FIG. 4B.

FIG. 4B is a circuit diagram of an illustrative embodiment for one ofthe banks of capacitors 430 utilized as part of variable capacitance416. FIG. 4B includes a fixed capacitor 450 that represents parasiticcapacitance plus any desired fixed capacitance. Discrete capacitancevariations are achieved through a plurality of selectively activatedcapacitor and transistor pairs. FIG. 4B shows a first of thesecapacitor/transistor pairs, as capacitor 452 connected between ground442 and the signal line 440 through the drain and source terminals of anNMOS transistor 462. NMOS transistor 462 acts as a switch to add in orleave out the capacitor 462 in the overall capacitance of the discretelyvariable capacitance 430. The “on” or “off” state of NMOS transistor 462is controlled by a first bit (B₀) of a digital control word suppliedfrom control logic as described further herein. Similarly, additionalcapacitors 454, 456, and 458 are connected to ground 442 through NMOStransistors 464, 466, and 468, respectively. The NMOS transistors 464,466 and 468 are controlled by bits B₁ . . . B_(N−1), B_(N), of a digitalcontrol word selecting the capacitance setting.

For the circuit depicted in FIG. 4B with simple capacitor/switchcircuits connected together in parallel, the total capacitance for thediscretely variable capacitance 430 is equal to the sum of all of thecapacitors having their respective switches in the “on” state. Note thatthe capacitors may all have the same value, providing for linearcapacitance stepping or the capacitors may be of different valuesproviding non-linear capacitance stepping. In the latter case, turningon one transistor will have a different effect than turning on anothertransistor. Numerous weighting schemes are possible, and the implementedweighting scheme depends upon the particular design considerationsinvolved. Additional details on the capacitor/switch circuits and theVCO can be found in U.S. Pat. No. 6,137,372, entitled “Method andApparatus for Providing Coarse and Fine Tuning Control for SynthesizingHigh-Frequency Signals for Wireless Communications”, which isincorporated herein by reference in its entirety.

In an exemplary embodiment, the variable capacitance 416 includesmultiple banks of capacitors 430 to provide coarse, medium andfine-grained control over the capacitance. The coarse bank of capacitorsincludes seven capacitor/switch circuits each controlled by one bit of aseven bit coarse grained digital control word. A medium bank ofcapacitors includes eight capacitor/switch circuits each controlled byone bit of an eight bit medium-grained control word. Finally, a finebank of capacitors is provided with 6 capacitors/switch pairs controlledby a six bit digital control word. The coarse, medium, and fine banks ofcapacitors allow control over total capacitance with increasinggranularity. In an exemplary embodiment, the capacitors in the coarsebank are 300 femtoFarads each, in the medium bank, 38 femtoFarads and inthe fine-grained bank, 9 femtoFarads. The use of the variablecapacitance 416 to acquire frequency of an input data stream without theuse of a reference clock is described further herein.

Before a description of the use of the architecture shown in FIG. 2 inreferenceless frequency acquisition is described in more detail, adescription of the operation of the phase detector 210 is provided tobetter understand use and operation of the augmented phase detectordescribed herein.

FIG. 5 depicts a common phase detector that is generally well known inthe art. In this circuit, a register 510 samples the input data signalconveyed on node 512 when clocked by the sample clock signal conveyed onnode 514. The sample clock signal conveyed on node 514 is the recoveredclock from the input data signal. XOR gate 520 generates on its outputnode B a variable-width pulse of duration generally equal to the time bywhich an input data signal transition leads the corresponding transitionon node A, the output of register 510, which is controlled, of course,by the recovered clock conveyed on node 514. The delay block 530 isincluded to compensate for the clock-to-Q delay of register 510. Whenthe data clock is correctly aligned to the input data signal (i.e., dataclock transitions at precisely the mid-point of the data bit-intervals),the register 510 generates on its output node A, a signal thatreplicates the input data signal, but delayed by one-half period of thedata clock, and the pulse on node B is of a duration exactly equal toone-half period of the data clock.

The latch 540 generates on its output node 542 a signal which replicatesits input signal delayed by one-half period of the recovered clock. As aresult, the second XOR gate 522 generates on its output node C a pulsewith a duration that is equal to one-half of the period of the recoveredclock. The fixed-duration pulse signal conveyed on node C is subtractedfrom the variable-width pulse signal conveyed on node B by summing block550 to generate a phase error signal used to adjust the recovered clock.

Operation of the phase detector shown in FIG. 5 is illustrated by thetiming diagram shown in FIG. 6. When the data clock is correctlyaligned, as illustrated in FIG. 6, both pulse signals have equalduration, and the summing block 550 generates a zero-valued net errorcurrent. If the input data transition arrives too early, the pulsesignal on node B is longer than the pulse signal on node C, and a neterror current is generated by the summing block 550. The remainder ofthe PLL is arranged to respond to the error current to adjust the phaseof the data clock (i.e., the VCO output).

While FIG. 5 shows single-ended logic blocks and signals, in practicesuch circuits are typically implemented using fully differentialcircuitry, which provides enhanced noise immunity, better speed, andmore consistent delays which are independent of data state. Moreover,many of the circuit blocks, such as the summing block 550, are moreeasily implemented and achieve better matching of currents whenimplemented differentially, thereby resulting in lower offsets.

As described previously, one method of determining if the VCO iscorrectly locked to the clock embedded in the input data stream is tocompare the recovered clock to a reference clock. If a reference clockis unavailable, another approach has to be used both for initiallysetting the VCO frequency and for determining if lock has been achieved.As described further herein, frequency detection can be accomplished bydetecting whether transitions fall into a predetermined phase zone (alsoreferred to herein as a “forbidden zone”) of the data clock used in thephase detector. The “forbidden zone” is a predetermined portion of theperiod of the sample clock used in the phase detector. When the PLL isin-lock, transitions typically will not fall into this zone. When thePLL is out-of-lock, transitions will often fall into the zone. Byperforming a test over many trials to determine whether data transitionsin the input data stream are falling into the forbidden zone, it can bestatistically inferred whether the PLL is in-lock.

Referring again to FIGS. 5 and 6, the first register 510 of the phasedetector 500 samples the data at 0 degrees, i.e., the leading edge ofthe recovered clock used to sample the data. Thus, data transitions ofthe input data stream ideally occur around 180 degrees with respect tothe recovered clock to maximize the likelihood that a transition isdetected given the existence of jitter and phase offset. FIG. 7illustrates the preferred placement of data transitions 701 assuming lowjitter. FIG. 8 illustrates the influence of phase offset on theplacement of data transitions with respect to the phase of the recoveredclock being used to sample the data in the first register of the phasedetector 500 in FIG. 5. As can be seen in FIG. 8, the transitions havemoved clockwise with respect to the recovered clock. FIG. 9 illustratesthe effect of phase offset and increased jitter amplitude. The datatransitions can be seen to be more distributed in the phase of therecovered clock. If the jitter is sufficiently high, it can lead to biterrors such as transition 703.

If the PLL is trying to acquire the frequency of a clock embedded in aninput data stream and no reference clock is available, the effect offrequency offset is one approach that can be used to determine whetherthe frequency is too high or too low and adjust the sample clockfrequency accordingly. Frequency offset is the difference in frequencybetween the input data stream and the sample clock, and causes the datatransitions in the input data stream to travel from quadrant to quadrantas shown in FIG. 10. The speed at which the transitions travel fromquadrant to quadrant increases as the frequency offset increases. Thedirection of travel (clockwise or counterclockwise) indicates whetherthe frequency error is positive or negative. The number of transitionsthat occur in a quadrant before the transitions enter a new quadrant isa function of the magnitude of the frequency offset and the datatransition density (assuming, e.g., a non-return to zero (NRZ)encoding).

Thus, one method of frequency detection is to determine the frequencyoffset by determining the order of the quadrants that the datatransitions go through. That order indicates the direction of travel ofthe transitions and thus can be used to alter the VCO output positivelyor negatively according to the sign of the frequency error. However,that approach has several disadvantages. One problem is that the rangeof frequency detection is limited. The range of frequency detection isin part a function of the data transition density. A low transitiondensity increases the difficulty in resolving the direction of travel ofthe transitions. Additionally, the range of frequency detection islimited because it is a function of quadrant sizes. A larger frequencyoffset can cause larger intervals between transitions in the quadrants,which can also increase the difficulty in resolving transitiondirection. In addition, quadrature phases are required for the largestfrequency detection range, which can be difficult to implement with anLC oscillator.

In view of those limitations, another approach can be used to detect thefrequency of an input data stream that overcomes those problems.Referring to FIG. 11, a small phase window or “forbidden zone” 111 canbe defined. Any transitions, such as transition 112, that occur in thatphase window 111, are assumed to be bit errors. The phase window 111(also shown as Δφ) should be placed on the side of the zero degree markthat is farthest from the placement of the data transitions that occurdue to phase offset. There may be a systemic offset due to, e.g.,circuit delays, as well as random offset. The systemic offset in oneembodiment rotates the placement of the data transitions clockwise fromthe 180 degree mark. Thus, the forbidden zone in such an embodiment isoffset in a clockwise direction from the zero degree mark as shown inFIG. 11. In other embodiments, the phase zone can reside on the otherside of the zero degree point.

If the PLL has not locked to the frequency of the input data stream,data transitions will eventually fall into the forbidden zone. Thatinformation can be used to determine whether the PLL has locked, and ifit has not, adjust the output of the VCO until it does. The “forbiddenzone” approach requires a circuit that detects transitions that occur inthe forbidden zone. FIG. 12 A illustrates a simplified augmented phasedetector circuit 1200 that detects transitions in the forbidden zone.Comparing the phase detectors in FIG. 12A and FIG. 5, it can be seenthat the top half 1201 of the phase detector circuit 1200 issubstantially identical to the phase detector 500 shown in FIG. 5 andoperates conventionally to generate an error signal from summer 550indicating the phase error between the data in signal 512 and the clocksignal 514 supplied from the VCO. In addition to the conventional phasedetector 1201 illustrated, the phase detector of FIG. 12A furtherincludes forbidden zone circuitry 1202 that detects transitionsoccurring in the forbidden zone of the clock signal 514. That circuitryincludes a delay element 1203 that functions to delay the clock 514 byan amount corresponding to the size of the phase window. In an exemplaryembodiment, the delay element 1203 delays the clock by 50-60picoseconds. The amount of delay depends on such factors as the size ofthe phase window desired, the bit rate of the input data stream, and thebit error rate of the input data stream. The delayed clock 1204 issupplied to registers 1205 and 1206 and to the latch 1207. The XOR gate1209 generates a pulse that indicates the difference between the retimeddata on node 542 and the delayed data supplied from latch 1207. Notethat the latches 540 and 1207 are configured to pass data when the clockis low.

FIG. 12B illustrates an alternative embodiment in which the clock forthe register 510 is delayed rather than the clock for register 1205.Note also that rather than delaying the clock in either path, the datain either path may be delayed.

The operation of the circuit of FIG. 12A is illustrated in the timingdiagrams 13A and 13B. In FIG. 13A the data is transitioning in the ideallocation at 180 degrees with respect to the phase of the clock signal514 (shown as CLK in FIG. 13A and 13B). The output from the registersand latches are shown to create a pulse on node F from XOR gate 1209.However, the duration of that pulse is short and therefore is notclocked into register 1206 by the rising edge of the delayed clock (DELCLK). Thus, no error is detected and the SR latch 1210 is not set.

Referring to FIG. 13B, the operation of phase detector 1200 isillustrated for the case where data transitions do occur in theforbidden zone. When that occurs, the transition on Data In occurs afterthe rising edge of CLK conveyed on node 514 and thus is not clocked intoregister 510. The delayed clock 1204 (DEL CLK in FIG. 13B) is delayedsufficiently to catch the transition and thus the transition is storedin register 1206. As the transition or lack thereof propagates throughthe latches 1207 and 540, XOR gate 1209 generates a pulse output on nodeF that is clocked into register 1206 on the rising edge of the delayedclock. That in turn causes the SR latch to be set. The SR latch remainsset indicating the existence of a bit error until control logic,described further herein, causes the SR latch to be reset so it can beused to detect another bit error.

As described more fully herein, the ability to detect the forbidden zonetransitions allows the VCO output to be adjusted until transitions inthe forbidden zone occur at a rate below the allowable bit error rate.The forbidden zone approach requires only a small delay element, e.g.,utilizing a buffer, rather than needing to detect in which quadrature ofthe VCO output clock phase a transition occurs. In addition, thefrequency capture range is not limited by those limitations associatedwith quadrature detection. The quadrature detection approach is “soft”in terms of evaluating whether the PLL is in-lock. The VCO is simplynudged in a certain direction when data shifts through quadrantboundaries. Thus, with a large frequency offset, the quadraturedetection method may drift. In contrast, the forbidden zone approachdescribed herein makes “hard” decisions. A frequency offset is evaluatedand then a VCO setting is either rejected or accepted. One cannot driftthrough the same settings as in the quadrature approach, until all VCOsettings have been tried.

The probability of entering the forbidden zone in out-of-lock conditionscan be made approximately the same for all frequency offsets. Thatfacilitates a clock and data recovery circuit that can operate at a widerange of frequencies. In addition, the detection circuitry and controllogic can be mostly built in digital logic allowing for easyimplementation and low gate count. That can be particularly advantageousin mixed signal technologies. Note that in the forbidden zone approach,high bit error rates can result in the inability to lock. That willbecome more obvious as the approach is described in more detail. Incontrast, the traditional quadrature approach may be fairly insensitiveto bit error rates.

One way to understand the forbidden zone approach is to understand thestatistical behavior associated therewith. That statistical behavior canbe explained using Bernoulli processes. Referring to FIG. 14, aBernoulli process is a series of independent Bernoulli trials, where theoutcome can be a one or a zero. The probability that x_(i)=1 is definedas p and the probability that x_(i)=0 is therefore 1−p. Define k as$k = {\underset{i = 1}{\sum\limits^{n}}{x_{i}.}}$The probability that k=k₀ is:${p_{k}\left( k_{o} \right)} = {\left( \frac{n\quad!}{{\left( {n - k_{o}} \right)!}\quad{k_{o}!}} \right){p^{k_{o}}\left( {1 - p} \right)}^{n - k_{o}}}$

A better numerical implementation of that equation is:$\frac{n\quad!}{{\left( {n - k_{o}} \right)!}\quad{k_{o}!}} = {\underset{i = 1}{\prod\limits^{k}}\frac{n - k + i}{i}}$

If a probability of occurrence of an event is p, then the expected valueof the first occurrence is:E(l ₁)=1/p

With that background, assume that x_(i)=1 if any transitions fall intothe forbidden zone and that x_(i)=0 if no transitions fall into theforbidden zone. A measurement period T is chosen over which to measurewhether any transitions fall into the forbidden zone. For example, T maybe 1024 times the period of the rate data. The measurements over periodTare repeated over n intervals of time. The number of intervals k iscounted in which at least one transition fell into the forbidden zone:$k = {\underset{i = 1}{\sum\limits^{n}}x_{i}}$

That value k is then compared to a critical count value k_(c), and ifk>k_(c), the PLL is declared out-of-lock. If k<k_(c), then the PLL isdeclared in-lock.

Because the PLL is being declared in-lock or out-of-lock based ondetection of transitions in the forbidden zone, there exists thepossibility of falsely declaring the PLL being out-of-lock because of ahigh bit error rate (BER) causing transitions to occur in the forbiddenzone. There also exists the possibility of falsely declaring lockbecause sufficient transitions did not fall into the forbidden zone overa period of time. Remember that in a preferred embodiment, the data isencoded in an NRZ format or an equivalent and therefore transitiondensity can vary based on data patterns. The probability of falselydeclaring the PLL to be in-lock or out-of-lock should be sufficientlysmall as to be tolerable in the system in which the forbidden zonedetection approach is employed. The specific equations for suchprobabilities are provided herein.

Note that the size of the forbidden zone can vary as a percentage of theentire period, and that a larger size has the effect of lower jittertolerance since a larger number of data transitions caused by jitterwill be determined to be bit errors.

Assume that transitions occur in the forbidden zone according to the biterror rate (BER). Calculate P(x_(i)=0)=(1−BER)^(m), where m is thenumber of bits in the measurement period, or m=(data rate)(T).Of course, P(x _(i)=1)=1−P(x _(i)=0).

Assume that transitions are uniformly distributed across the phaserange. That is illustrated in FIG. 15. That is roughly true if theperiod of frequency offset<period of measurement. If that is true, theprobability that a transition does not occur in the forbidden zone overa period T is given by,${{P\left( {x_{i} = 0} \right)} = \left( \frac{{2\quad\pi} - {\Delta\quad\Phi}}{2\pi} \right)^{m}},$where m is the number of transitions in the measurement period andm=(data rate)(T)(transition density). Of course,P(x_(i)=1)=1−P(x_(i)=0).

The desire to achieve a uniform transition distribution when the clockand data recovery (CDR) circuit is out-of-lock leads to the period offrequency offset being less than (or equal to) the period of measurement(evaluation interval). In an exemplary embodiment suitable for meetingSONET specifications, the minimum period of frequency offset is given bythe frequency lock-in range of the CDR. For a typical SONET transferspecification, the frequency lock-in range is on the order of 1000 partsper million (ppm). Thus, the minimum offset frequency is approximately(2.5 GHz×1000×10⁻⁶)=2.5 MHz. That is, the CDR should be able to acquirea frequency that is within 2.5 MHz of the 2.5 GHz data rate. In such anapplication, the measurement period T=1/(2.5 MHz)=0.4 microseconds.

The probability that x_(i)=1 (incorrect decision) when PLL is actuallyin-lock is given by,P(x _(i)=1)=1−(1−BER)^(m),where m is the number of bits in the measurement period, that is m=(datarate)T.

The probability that that x_(i)=1 (correct decision) when PLL is notin-lock is given by,${{P\left( {x_{i} = 1} \right)} = {1 - \left( \frac{{2\pi} - {\Delta\Phi}}{2\pi} \right)^{m}}},$where m is the number of transitions in the measurement period,m=(data rate)T(transition density)

The probability of falsely asserting out-of-lock when PLL is in-lock isgiven by${p\left( {k > k_{c}} \right)} = {\underset{i = {k_{c} + 1}}{\sum\limits^{n}}{\left( \frac{n\quad!}{{\left( {n - i} \right)!}\quad{i\quad!}} \right){p^{i}\left( {1 - p} \right)}^{n - i}}}$wherep=1−1(1−BER)^((data rate)T)

The probability of falsely asserting lock when the PLL is out-of-lock isgiven by,${{p\left( {k<=k_{c}} \right)} = {\underset{i = {n - k_{c}}}{\sum\limits^{n}}{\left( \frac{n\quad!}{{\left( {n - i} \right)!}\quad{i\quad!}} \right){p^{i}\left( {1 - p} \right)}^{n - i}}}},$where $p = \left( \frac{{2\pi} - {\Delta\Phi}}{2\pi} \right)$

Exemplary curves for various bit error rates and various sizes of theforbidden zone will be provided after embodiments of the augmented phasedetector circuit and the control circuitry used to adjust thecapacitance of the LC oscillator, used for the variable oscillator inone embodiment of the invention, are examined in greater detail.

Referring now to FIG. 16, an exemplary high level flow diagram is shownthat illustrates an embodiment of the control structure utilized toacquire a frequency. The state machine of the exemplary clock and datarecovery circuit (CDR) begins in state 0, which is entered as a resultof a power on (or other) reset or because of assertion of a lock toreference (LTR) signal. The lock to reference signal causes the CDR tolock to a reference clock (if available) or to stored capacitance valuesof the VCO as described further herein. In state 0, the loss of lock(LOL) signal is asserted indicating that the clock and data recoverycircuit has not yet locked. When neither reset nor LTR is beingasserted, the CDR enters state 1 and begins the task of acquiring theinput frequency.

In state 1, an impedance setting is chosen. Prior to, or on enteringstate 1, several initialization steps may be taken. For example, thefine capacitor setting may be zeroed out to allow the fine capacitorsetting to be swept for each medium/coarse setting. In addition, thedigitally controlled capacitors may be set at their midrange to preparefor frequency acquisition. Other details of the control logic aredescribed further herein.

Referring now to FIG. 17, one preferred approach to the steppingalgorithm used to vary the capacitor settings is illustrated. As shownin FIG. 17, the capacitor setting is varied gradually above and below acenter capacitance value. With each step of the algorithm, thecapacitance is varied with increasing amplitude above and then below theinitial setting. That has the advantages of minimal frequency deviationafter first lock is achieved. In addition, a fast acquisition isachieved when the desired setting is close to a current setting.Further, the probability of locking onto a harmonic of the desiredfrequency is reduced. In addition, that approach avoids large frequencydeviations, which may be undesirable from a customer's viewpoint. In oneembodiment, the approach of varying the capacitance above and below acenter value is applied only to the coarse and medium capacitancesettings. The fine capacitance settings are swept in a linear fashionfor each medium/coarse setting. In one embodiment, the control logic canstep through 336 different capacitance settings, including 6 finesettings, 7 course settings, and 8 medium settings. Note that the use ofthe stepping algorithm shown in FIG. 17 is exemplary only. Otherapproaches, including a completely linear approach, may also be used. Inaddition, the use of fine, medium and coarse capacitance settings isexemplary. Additional settings or fewer settings may be used accordingto system requirements.

Referring again to FIG. 16, once one of the capacitor settings isselected in state 1, the state machine moves to state 2 in which theselected capacitor setting is evaluated for bit errors. On enteringstate 2, a bit error counter and a trial counter are initialized. Biterrors are determined to be those transitions that occur in theforbidden zone. The evaluation in state 2 is intended to provide a fastindication of lock or lack thereof after relatively few trials (only 16trials lasting approximately 6.6 μs). A trial is an evaluation period onperiod of measurement described previously lasting for at least theminimum period of the frequency offset.

The risk associated with few trials is that there will be a false lock.The approach described in FIG. 16 uses initially few trials followed bysuccessively larger number of trials. If a false lock occurs, the nextseries of longer trials should detect the false lock. Note that the fastevaluation (16 trials) is sensitive to a long string of transitionlessbits.

In one embodiment, the control logic is being clocked at a ratesignificantly slower than the data rate. For example, the data rate maybe 2.488320 GHz (OC-48), while the state machine operates at a clockrate of data rate/1024 or 2.43 MHz. For each state machine clock cycle1024 bits are transmitted and evaluated by the forbidden zone detectioncircuit. Note that the time period of each evaluation cycle (or trial)corresponds to the 0.4 microseconds measurement period T describedabove.

If a transition does occur in the forbidden zone during the measurementperiod T, the bit error counter increments to record the error. Thetrial counter counts the number of trials over which to evaluate theexistence of a bit error during the 1024 bit times. In the illustratedembodiment, 16 evaluation cycles or trials are performed in state 2,which corresponds to approximately 6.6 microseconds. If after 16evaluation cycles the bit error count is 16, meaning that at least oneforbidden zone transition occurred in each of the 16 evaluation cycles,the state machine returns to state 1 to step the digital capacitorsettings and thereby adjust the capacitor settings in accordance with,e.g., the stepping algorithm described in FIG. 17. If however, after 16evaluation cycles are completed, the bit error count is less than 16,then the state machine enters state 3.

State 3 provides a longer period of evaluation than state 2, which helpsidentify false lock conditions. On entering state 3, the bit errorcounter and the trial counter, which respectively count the number ofdetected bit errors and the number of evaluation cycles utilized in theparticular state, are both reinitialized, e.g., set to zero. In theillustrated embodiment, 48 cycles are evaluated in state 3. If bit errorcount is 43 or greater, indicating that lock has not been achieved, thecontrol logic returns to state 1 to step the capacitance settings. Inone embodiment, the evaluation of lock in state 3 is robust against 4000transitionless bits and a maximum bit error rate of 1×10⁻³. In theillustrated embodiment, the state 3 evaluation takes approximately 19.6microseconds given the OC-48 data rates described earlier. If, howeverat the end of those 48 cycles, the bit error count is less than apredetermined number (43 is the illustrated embodiment), the controllogic enters state 4.

State 4 provides a longer period of evaluation than state 3. The biterror counter and the trial counter, which respectively count the numberof bit errors and the number of evaluation cycles utilized in theparticular state, are both initialized. In the illustrated embodiment,512 cycles are evaluated in state 4. At the end of those 512 cycles, ifthe bit error count is less than a predetermined number (497 in theillustrated embodiment), the control logic enters state 5. If however,the bit error count is 497 or greater, indicating that lock has not beenachieved, the control logic returns to state 1 to again step thecapacitance settings. The evaluation of lock in state 4 is robustagainst 8000 transitionless bits and a bit error rate of up to 2×10⁻³.In the illustrated embodiment, the state 4 evaluation takesapproximately 209.7 microseconds, given the rates described earlier.While one preferred embodiment utilizes successively longer testingstates, other embodiments may include only one or fewer states. Thatwould entail determining lock after, e.g., 1000 trials.

State 5 is a locked state, and loss of lock (LOL) is deasserted in state5 to indicate that lock has been achieved. The state machine remains inthe locked state evaluating bit errors. The bit errors may again beevaluated over e.g., 512 cycles. At the end of those 512 cycles, if thebit error count is less than a predetermined number (e.g., 497 in theillustrated embodiment), the control logic remains in state 5. Ifhowever, the bit error count is 497 or larger, the state machine returnsto state one and asserts loss of lock. In the locked state, errormonitoring may be selectively enabled. In addition, the capacitorsettings for the VCO can be saved (and the accumulator value from theintegrating path 240). If lock is lost, those saved values may be usedto control the VCO output to output a clock that was recently locked tothe input data stream.

With that understanding of how bit errors (transitions in the forbiddenzone) are used to determine whether the PLL is locked, an embodiment1800 of an augmented phase detector that determines when transitionsoccur in the forbidden zone is illustrated in FIG. 18. The circuit issimilar to the phase detector illustrated in FIG. 12, except there areextra latch stages 580, 581 and 1409 and 1411. In the embodimentillustrated in FIG. 18, the latch 540 insulates the earlier signalentering the XOR gate 522 from variations in the timing at node Aresulting from varying input data timing (i.e., variations in clock-to-Qtiming of register 510 as a function of its input data setup time). Thelatches 581 and 1811 ensure that the retimed data is not out of phasewith the input data (half a cycle off) and may also be preferable from aloading perspective.

The embodiment illustrated in FIG. 18 also provides a variable lengthdelay, which translates into a phase window appropriate for differentdata rates. The phase window is a portion of the period of the recoveredclock conveyed on node 514. In one embodiment, when the delay path 1815is selected by selector 1803, the delay is approximately 50-60picoseconds, which corresponds to an approximately 15% phase window fora 2.5 GHz data rate. However, if a slower data rate is being used, extradelay can be selected by selecting the delay path incorporating extradelay 1801. A rate select signal indicating the data rate frequency maybe used for the selector signal 1805. While only two delays areillustrated, multiple delays may be selectable to accommodate a widerrange of data rates.

As also illustrated in FIG. 18, when a bit error is indicated on node1820, the control logic resets the SR latch using the reset signal 1821.

FIG. 19 illustrates another embodiment of an augmented phase detectorthat determines when transitions occur in the forbidden zone. Thecircuit is similar to the augmented phase detector illustrated in FIG.18. The augmented phase detector 1900 includes the phase detectorportion 1901 and a bit error detector 1903. The primary differencebetween the augmented phase detector 1900 and the augmented phasedetector 1800 is that the delays are implemented differently. Theembodiment illustrated in FIG. 19 is intended to support multiple clockfrequencies including the various SONET data rates includes OC-3(155.520 Mbs), OC-12 (622.080 Mbs), OC-24 (1.244.16 Mbs), and OC-48(2,488.320 Mbs). There are two delay paths in the bit error detector1903 based on clock frequency. The clocks for OC24/OC12/OC3 utilize adelay path 1905, while the OC-48 clock utilizes the delay path 1907. Amultiplexer 1909 selects the appropriate lower speed clock for delaypath 1905. Multiplexer 1911 selects between delay path 1907 and 1905.The data conveyed on node 512 is clocked into register 1913 and latch1915. The output from the selected delay path is latched into latch1921. The output from latch 1921 and latch 540 are compared in XOR gate1923. Note that in a preferred embodiment, all signals shown in FIG. 19are differential except for rate3, rate48, ber_reset, and ber_detect. Asmore fully described herein, the delay in the OC48 path (multiplexer1911=1) derives its delay as the interpolation between clk48 and phclkas described more fully herein. Note that the phclk conveyed on node1924 is determined according to the particular clock frequency beingsupported.

At the beginning of a test cycle (e.g., 1024 data bit times), theber_reset signal conveyed on node 1930 is set to the opposite binaryvalue of the ber_detect signal conveyed on 1931. At the end of the testinterval, if ber_detect equals ber_reset, then an error was not detectedby comparator 1923 and register 1933 is not clocked and ber_reset andber_detect remain at different values. If on the other hand, an error isdetected by XOR gate 1923, register 1933 is clocked causing theber_reset to be clocked into register 1933, converted to a single endedsignal in differential to single ended converter 1935, and output onnode 1931 as the ber_detect signal.

FIG. 20 illustrates the implementation of the delay associated with theOC3 clock and the generation of the clocks for OC24, OC12 and OC3 modeof operations. All the illustrated signals are differential. The OC-48clock (clk48) conveyed on node 2001 is divided in half to create clk24conveyed on node 2003, and further divided to create clk12 conveyed onnode 2005.

The phclk3_pre is twice the OC3 (clk3) frequency conveyed on node 2007.The clock phclk3_pre is supplied on node 2009 to multiplexer 1909. Thecritical edge of phclk3_pre is three half cycles of clk48 (ideallyapproximately 600 picoseconds but less in practice due to loading).

Note that, phclk3_pre could also be produced by tapping before the latch2011 producing clk3. The location of tapping to generate phclk3_pre maydepend on loading issues. In the particular implementation shown in FIG.20, the amplifier 2013 is utilized to minimize the impact of loading.The use of the amplifier lowers the delay between phsclk and phclk3_preand therefore reduces the forbidden zone. That reduction in theforbidden zone may be undesirable.

FIG. 21 illustrates implementation of the delay associated with OC24 andOC12 clocks. Clk48, clk24, clk12 and clk3 are gated by the respectiveclock select signals rate48, rate24, rate12 and rate3 signals in buffers2101, 2103, 2105, and 2107. Note that all signals shown in FIG. 21 aredifferential. Phclk_pre12 ideally precedes phclk by half a clk48 period(approximately 200 picoseconds) for OC24 and OC12 clocks. Phclk_pre12 isused for both OC24/OC12. The clock phclk, conveyed on node 1924 to clockthe phase detector 1901 is selected by the selector circuit 2100 shownin FIG. 21. For OC48, phclk is delayed from clk48 by the propagationdelay of one buffer (approximately 60 picoseconds). However, in someembodiments, a one buffer delay provides a forbidden zone that is toolarge.

Accordingly, interpolation between the clock phclk conveyed on node 1924and the clock clk48 conveyed on node 1925 is utilized to achieve asmaller delay and thus a smaller forbidden zone. A smaller forbiddenzone gives better jitter tolerance and more accurate ber measurements.In one embodiment, the forbidden zone is approximately 0.1 UI.

FIG. 22 illustrates interpolating between two clocks to achieve ashorter delay than can be achieved by utilizing one buffer. FIG. 22illustrates a differential latch 2200 which can be cascaded two in a rowto form a register with interpolated sample time such as thedifferential register 1913 (shown in FIG. 19). Assume clk2 is generatedby passing clk1 through a differential buffer with a delay ofapproximately 60 picoseconds. The sample time of the register isdetermined by the interpolation of the clk1 and clk2 transition edges asset by the relative values of the bias currents Ibias1 and Ibias2.Assume I_(bias1)+I_(bias2)=I_(bias). The sample time is determined bythe I_(bias1) and I_(bias2). For example, if I_(bias1)=I_(bias), andI_(bias2)=0, the transition edge of clk1 determines the sample time. IfI_(bias1)=0, and I_(bias2)=I_(bias), the sample time is determined bythe transition edge of clk2. Finally, if I_(bias1)=I_(bias2)=I_(bias)/2,the sample time is approximately half way between the transition edgesof clk1 and clk2. Thus, the sample time, and thus the delay and thewidth of the forbidden zone, can be narrowed or lengthened by adjustingthe bias currents. Note that the bias current I_(bias) is set accordingto the voltage drop desired across resistor loads R_(L).

FIG. 23 shows a block diagram of an exemplary clock and data recoverycircuit that can advantageously exploit the augmented phase detectorsand associated control logic described herein (particularly FIG. 19).The clock and data recovery circuit shown in FIG. 23 includes a controlcircuit 2301 for referenceless frequency acquisition. The controlcircuit 2301 operates in accordance with the state machine shown in FIG.16. The control circuit receives a bit error detect signal conveyed onnode 1931 and supplies the reset detector signal on node 1930. Asdescribed previously, when the bit error detect signal andreset-detector signal are at different values, a bit error has occurred.

Aside from receiving the bit errors and resetting the detect logicassociated therewith, the control circuit also outputs the capacitorsettings used by the VCO 260. As described previously, the VCO 260receives the control signal V_(g) conveyed on node 252 that adjusts theanalog capacitance 410 (FIG. 4). In the illustrated embodiment, thecontrol circuit 2301 supplies the coarse, medium, and fine capacitorcontrol settings for the digitally controlled capacitors 416 (FIG. 4) onnode 2310. Node 2310 may be implemented as a multi-bit value supplying,e.g., control values indicating 7 different coarse settings, 8 differentmedium settings and 6 fine settings for a total of 336 possiblecapacitor settings.

The control logic also provides a variety of control signals useful invarious aspects of the disclosed embodiments. For example, the controllogic supplies the mid_rail signal 2304 to the delta-sigma modulator 212to cause its output to be set to its midpoint when the PLL is operatingin a lock to reference mode (state 0). If loss of lock has occurred,asserting the mid_rail signal 2304 zeros the feedforward path 230. Thefeedforward high bandwidth (ff_high_bw) signal conveyed on node 2306 tothe gain block 232 of the feedforward path 230 functions as a bandwidthcontrol signal to select either a higher or lower open loop bandwidth.During acquisition of the frequency of the input data stream, (states1-4) the feedforward high bandwidth signal is set to maximize the gainto provide a wider bandwidth and thus better lock-in range. The gain isthen reduced once confidence of lock is higher or once lock is achievedto provide better performance since a wide lock-in range is no longerneeded. The center accumulator (center_accum) signal adjusts the up/downcounter in the accumulate block 244 to its center value. When asserted,the integrating path is essentially shut off. An integrating path highbandwidth control signal (int_high_bw) is conveyed on node 2310 toadjust the filter bandwidth provided in filter 248. The center softswitch (center_soft_sw) control signal conveyed on node 2312 to softswitch control circuit 2314 causes the soft switch control to output acenter capacitance value during acquisition of the frequency.

In an exemplary embodiment, in state 2 of FIG. 16, center_accum=1,ff_high_bw=1, int_high_bw=1, and mid_rail=0, with a 1 indicating anasserted signal. With control signals at those values, the integratingpath is turned off and the feed focused path is set for high gain. Instate 3, center_accum=0, ff_high_bw=1, int_high_bw=0, thus enabling theintegrating path. In state 4, center_accum=0, ff_high_bw=1, andint_high_bw=0. In the locked state the feedforward high bandwidth signalis deasserted to reduce gain.

Note that the augmented phase detector circuit 1900 in the illustratedembodiment is receiving a clock signal conveyed on node 1924 that in theillustrated embodiment supports SONET OC-48/OC-24/OC 12/OC3 data rates(approximately 2.5 GHz for OC-48). The delta-sigma modulator receives aclock at half that rate. The control circuit 2301 receives a clock thatis divided down by 1024. One advantage of operating the control circuit2301 at a much slower clock rate is that it runs slowly enough that itcan be easily synthesized by readily available synthesis tools, whichresults in design savings. In addition, a slower clock rate can savepower during operation.

There are two basic errors that can occur utilizing the forbidden zonebit error detection. The first potential error is asserting that the PLLis out-of-lock when it is in fact locked. The probability of thathappening depends upon the number of trials that occur as well as thebit error rate of the transmitted data. Referring to FIG. 24A, anexemplary graph illustrates the probability of falsely assertingout-of-lock with various bit error rates (BER) versus various bit errorcounts for 16 trials. Each of the curves shown in FIG. 24A correspondsto a different bit error rate. The first curve corresponds to a biterror rate of 1×10⁻⁴. As the bit error count increases from 4 to 11, theprobability of falsely declaring out-of-lock declines from 1×10⁻² to1×10⁻⁹. That is, the probability declines from one in a hundred to onein a billion. Note that as the bit error rate increases, the probabilityof falsely declaring out-of-lock also increases. In other words, ifthere are more bit errors, they are more likely to fall in the forbiddenzone resulting in the false declaration of being out-of-lock.

A second potential error that can occur using the forbidden zoneapproach described herein is to falsely declare the PLL is locked whenin fact it is out-of-lock. One parameter that effects the falsedeclaration of lock is the size of the forbidden zone. If the size ofthe forbidden zone is increased, more data transitions will occur in thehidden zone and thus more data transitions will be considered bit errorsand thus the probability of falsely declaring lock should decrease witha larger forbidden zone. FIG. 24B shows the probability that lock isfalsely declared versus count and width of the forbidden zone. When theforbidden zone (ΔΦ/2π) is 10%, with a count of fourteen, the probabilityof falsely declaring lock is less than 1×10⁻¹⁶. However, for the samecount, when the forbidden zone is 5% of the period, the probability offalsely declaring lock is approximately 1×10⁻⁷. Thus, the probability offalsely declaring lock decreases with an increasing forbidden zone.

FIGS. 24A and 24B shows the probability for 16 trials, which correspondsto state 2 in FIG. 16. The probability curves for state 3 (48 trials) isshown in FIGS. 25A and 25B with the probability of falsely declaringout-of-lock shown in FIG. 25A for various bit error rates versus count.As would be expected, the probability of falsely declaring out-of-lockdecreases with an increasing count. Thus, with a BER of 1×10⁻³, theprobability of falsely declaring out-of-lock is less than 1×10⁻⁴ with abit error count of 42.

FIG. 25B shows the probability of declaring lock falsely for variousforbidden zone widths and counts. FIG. 25B shows that the probability offalsely declaring lock is decreased as the forbidden zone increases.

The probability curves for states 4 and 5 (512 trials) are shown inFIGS. 26A and 26B. FIG. 26A illustrates the probability of falselydeclaring out-of-lock versus count for various bit error rates. As wouldbe expected, the probability of falsely declaring out-of-lock decreaseswith increasing count. Thus, even with a BER of 2×10³, the probabilityof falsely declaring out-of-lock is less than 1×10⁻¹⁵ for a bit errorcount of 500.

FIG. 26B shows the probability of declaring lock falsely for variousforbidden zone widths and counts. FIG. 26B shows that the probability offalsely declaring lock decreases with an increased size of the forbiddenzone.

The forbidden zone detection circuitry used to determine whether or notthe PLL is locked can also be used to indicate the bit error rate oncethe PLL is locked. Thus, in the locked state, the state machine cancontinue to monitor the number of bit errors that occur during, e.g.,512 cycles. If after 512 cycles the number of detected bit errors isless than the predetermined threshold of approximately 500, then the PLLis considered to still be locked, the control logic remains in thelocked state, the bit error counter and trial counter are reset, and themonitoring continues for another 512 evaluation cycles.

If the bit error rate is above the threshold count value ofapproximately 500, the state machine determines the PLL to beout-of-lock and the state machine returns to state 1 and tries toreacquire lock. In trying to reacquire lock, the state machine can entera hold_vco_state prior to trying to reacquire lock In that state, theaccumulator state is maintained (center_accum is set to 0), thecenter_soft_sw is set to 0, the high bandwidth signals (high_bw) for thefeedforward and integrating paths are both set to 0 and mid_rail isasserted. An enable signal for the error monitor function is disabledsince the PLL is out-of-lock. The error monitoring function is onlyvalid while the PLL is in-lock. In addition, the control logic delays inthis state for 12.3 microseconds to allow for a loss of signal (LOS)exception to occur. That LOS exception can result in LTR being asserted.While in this initialization state, the VCO outputs a clock having afrequency based on the stored VCO settings. Note that the accumulatorsettings from several intervals ago may be used to control the VCOoutput.

One advantage of using the forbidden zone detection technique is that inthe locked state, if the bit error rate is below the threshold count(e.g., 501), the number of evaluation cycles having transitions in theforbidden zone can be used to indicate the actual bit error rate. In thelocked state, the various ranges of bit errors (a bit error being anevaluation cycle having one or more forbidden zone transitions) areassigned a digital value. FIG. 27A shows a table of one embodiment ofassigning a digital value corresponding to the number of trials(evaluation cycles) having one or more forbidden zone transition for 512trials in the locked state. That digital value can be stored in aregister. In one embodiment, the digital value of the error rate issupplied to a D/A converter that supplies an analog signal, typically ananalog current, on an output terminal of the integrated clock and datarecovery circuit (see FIG. 45) indicative of the bit error rate.Alternatively, if access is provided to that register, for instance by aserial communication port, that register can be read to provide anindication of the bit error rate. Note that when the PLL is not locked,the bit error rate indication can be invalidated by setting the outputterminal to a maximum or minimum value. Note that if the BER is 1, thena another bit error detection scheme is used to achieve betterresolution at low bit error rates.

More specifically, in one embodiment a secondary bit error rate counteris used to provide increased resolution by counting bit errors for alonger time period. More particularly, in one embodiment, the secondarybit error rate counter counts 256 locked-state cycles of 512 trialseach. Thus, the secondary bit error rate counter counts for 256×512trials. After each 512 trials, while in the locked state, the lowerorder bits (e.g., the three low order bits) of the BER counter are addedto a secondary BER counter. FIG. 27B illustrates the digital valuesgenerated based on the value of the sub bit error count according to oneembodiment of the invention.

Referring to FIG. 28, the probability curves for various bit error ratesshow why it is possible to infer the bit error rate from the count ofevaluation cycles having transitions in the forbidden zone. FIG. 28shows the probability that the measured count of transitions in theforbidden zone (the bit error count) is greater than the bit error ratefor 512 trials when the PLL is in-lock. As can be seen, for a bit errorrate of 1×10⁻⁴, the probability that the bit error count is greater thanthe bit error rate is negligible if the measured count is greater thanapproximately 50. For a bit error rate of 1×10⁻³, the probability thatthe bit error count is greater than the bit error rate is negligible ifthe bit error count is greater than approximately 328. Thus, the biterror count determined using the forbidden zone provide a good measureof the bit error rate, even for bit error rates as high as 3×10⁻³ whenthe bit error count is greater than approximately 489. For low bit errorrates additional resolution may be obtained using the sub bit errorcounter described in relation to FIG. 27B.

In addition to outputting an analog signal indicative of the bit errorrate, a BER alarm output terminal may be used to indicate that the biterror rate is above a programmable or fixed threshold value. Thatthreshold value may be determined by an analog signal (e.g., a voltage)supplied to a BER alarm level input terminal of the integrated circuit.The supplied analog voltage may be converted to a digital value andcompared with a calculated bit error rate. If the measured bit errorrate is above the BER threshold value, the BER alarm output terminalindicating the bit error rate is above the threshold value is set. Inone embodiment the bit error alarm threshold can be set to one of 64discrete values between error rates of 10⁻³ and 10⁻⁴ by applying avoltage to the BER alarm level input terminal between 500mV and 2.25V,corresponding to bit error rates of 10⁻³ and 10⁻⁴, respectively. Thatvoltage is then converted to the appropriate six bit digital value andcompared with the calculated bit error rate to determine whether toassert the BER alarm output terminal. FIG. 41 illustrates the BER alarmoutput terminal and the BER alarm level input terminal.

The bit error rate, determined as described above, every 512 trials, maybe used to control the BER alarm output pin. Alternatively, the biterrors may be counted and the alarm output controlled more frequently ifdesired. In one embodiment, the BER alarm output is evaluated everyapproximately 25.6 microseconds. If after 64 trials (at 2.5 MHz), theBER counter is above a critical count value corresponding to the BERalarm level, the BER alarm is asserted. In one embodiment, hysteresismay be selectable so that once the BER alarm is asserted, it isdeasserted after 128 or 256 trials. Alternatively, no hysteresis may beutilized. The hysteresis utilized may be programmed over a serialcommunication port, hardwired in the design or pin programmable.

Referring to FIGS. 29-31, exemplary simulations illustrate operation ofstepping the capacitance values of the VCO while trying to acquire lock.In the embodiments illustrated in FIGS. 29-31, there are assumed to be 7different coarse capacitance settings and 8 different medium capacitancesettings. In addition, fine frequency capacitance settings (6 in anexemplary embodiment) are swept for each of the coarse/medium settings.In the example shown in FIGS. 29-31, the top graphs indicate the coarsecapacitance settings, the middle graphs indicate the medium capacitancesettings and the bottom graphs indicate the cumulative coarse/mediumcapacitance values. The capacitance values are in femtoFarads. In FIG.29, the initial medium capacitance setting is 1 and the initial coarsecapacitance setting is 1. Note that the stepping of the coarse andmedium capacitance settings causes the total capacitance shown in thebottom graph to step successively more positively and negatively aroundthe initial capacitance value determined by an initial coarse and mediumsetting of one. After 20 different setting combinations, a minimumcapacitance value is reached and the capacitance value is incrementedfor the remainder of the possible capacitance values. After 56 differentsettings, the stepping of the capacitance values begins to repeat.

In FIG. 30, the initial medium capacitance setting is 3 and the initialcoarse capacitance setting is 4. The stepping of the coarse and mediumcapacitance settings causes the total capacitance to step successivelymore positively and negatively around the initial capacitance valuedetermined by the initial medium and coarse setting. After 40 differentcombinations of course and medium settings, a maximum capacitance valueis reached and the capacitance value is decremented for the remainder ofthe possible capacitance values. After 56 different settings, thestepping of the capacitance values begins to repeat.

In FIG. 31, the initial medium capacitance setting is 7 and the initialcoarse capacitance setting is 6, which are maximum values. Since theinitial setting is a maximum value, the stepping of the coarse andmedium capacitance settings causes the total capacitance to stepnegatively from the initial maximum capacitance value. After 56different setting combinations, the minimum capacitance value is reachedand stepping of the capacitance values begins to repeat.

While variable capacitance circuits may be particular useful in LC tankcircuit implementations of an oscillator, other embodiments may utilizedifferent types of oscillators, e.g., a ring oscillator. In addition,rather than adjusting the capacitance to acquire lock, the voltage orcurrent supplied to a variable oscillator may be adjusted to changefrequency until lock is achieved. As long as a control parameter can beadjusted for the particular oscillator used so its output frequency canbe varied in response to bit error detection techniques describedherein, the type of oscillator that can be used to practice theinvention can vary widely and include a wide variety of voltagecontrolled and current controlled oscillator circuits.

FIGS. 32-40 illustrate simulations of frequency detection under variousscenarios of jitter and transition densities for one embodiment of theinvention. The figures show the VCO input and feedforward output (seeFIG. 23) in volts. The figures also illustrate accumulator output, thebit error rate counter and the stepping of the fine capacitors settings(0-5). Once the accumulator output begins to become positive, lock isachieved. In FIG. 32, the transition density is assumed to be ½ with nojitter. In FIG. 33, the jitter is 5 Unit Intervals (UI) at 100 kHz. ThePLL performance can be seen to exceed the jitter tolerancespecifications proposed for SONET/SDH equipment by Bellcore GR-253-CORE,Issue 2, December 1995 and the ITU-T G.958. The x axis shown in FIGS.32-40 is in thousands of microseconds.

FIG. 32 shows that the fine capacitor settings were adjusted each timethe BER counter reached 16, indicating that lock was not achieved instate 2. After approximately 250 microseconds, the VCO has locked to theinput data stream and the fine capacitor settings are no longer beingswept, indicating that the coarse and medium settings are stable aswell. Note that the accumulator output shown indicates offsets from acenter value and not necessarily negative or positive numbers. Note alsothat the characteristics of the input data stream affect the BERcounter. For example, FIG. 36, the BER counter, while locked, indicatesa higher bit error rate than FIGS. 32-35.

Referring to FIG. 41, the various embodiments described herein areparticularly useful in a clock and data recovery (CDR) integratedcircuit 370, which is utilized for high speed serial communicationsystems in which timing information and data is extracted by the PLL 372from the data supplied on input terminals 371. Note that an analog biterror rate threshold signal, described previously, is supplied on inputterminal 373 as the threshold for the bit error rate alarm signalsupplied on output terminal 374, which is asserted when the measured biterror rate is above the threshold value. The CDR 370 also supplies a BERoutput terminal 376, which supplies an analog voltage indicating the biterror rate as previously described herein. Lock to reference (LTR) is anexternally provided signal causing the control circuit 2301 (FIG. 23) toremain in state 0. The rate select inputs may be used to select theprogrammable delay values (see FIGS. 18 and 19) in the augmented phasedetector utilized in PLL 372.

Thus, various embodiments have been described for locking to a clockembedded in an input data stream without the use of a reference clock.The technique to lock to the clock may also be used to monitor andreport the bit error rate. The description of the invention set forthherein is illustrative, and is not intended to limit the scope of theinvention as set forth in the following claims. Other variations andmodifications of the embodiments disclosed herein, may be made based onthe description set forth herein, without departing from the scope andspirit of the invention as set forth in the following claims.

1. A method of acquiring timing associated with an input data stream,comprising: detecting whether transitions of the input data stream fallinto a predetermined portion of a sample clock period of a sample clockutilized to sample the input data stream; and evaluating whether aphase-locked loop (PLL) has acquired the timing of the input data streamaccording to occurrence of transitions of the input data stream in thepredetermined portion of the sample clock period.
 2. The method asrecited in claim 1 wherein the evaluating further comprises determiningover a plurality of time periods, each of the time periods including anincreasing number of evaluation intervals, whether the PLL is locked tothe timing of the input data stream according to a number of evaluationintervals having one or more transitions in the predetermined portion ofthe sample clock period.
 3. The method as recited in claim 1, whereinthe sample clock is a clock recovered from the input data stream.
 4. Themethod as recited in claim 1, wherein the evaluating includes counting anumber of evaluation intervals that have at least one transition in thepredetermined portion of the clock period, generating a count indicativethereof and determining if lock is achieved according to the count. 5.The method as recited in claim 4 wherein the evaluation intervals are atleast as long as a minimum period of frequency offset.
 6. The method asrecited in claim 4, further comprising comparing the count to athreshold count to determine if lock is achieved.
 7. The method asrecited in claim 4, further comprising adjusting an output frequency ofa variable frequency oscillator circuit if it is determined that lock isnot achieved.
 8. The method as recited in claim 7, wherein the outputfrequency is adjusted by changing a variable impedance associated withthe oscillator circuit until lock is achieved.
 9. The method as recitedin claim 8 wherein the variable impedance is changed by adjusting atleast a portion of the variable impedance in increasing incrementsaround an initial impedance value.
 10. The method as recited in claim 9further comprising sweeping another portion of the variable impedancelinearly for each impedance setting resulting from adjusting at least aportion of the variable impedance in increasing increments around theinitial impedance value.
 11. The method as recited in claim 8, whereinthe variable impedance is a capacitance.
 12. The method as recited inclaim 11 wherein the oscillator circuit is a tank circuit including aninductive element.
 13. The method as recited in claim 7 wherein theoscillator circuit is a ring oscillator.
 14. The method as recited inclaim 8 wherein the oscillator circuit is a voltage controlledoscillator (VCO).
 15. The method as recited in claim 1 wherein thepredetermined portion of the clock period is adjacent to a clock edgeused to sample the input data stream.
 16. An integrated circuitcomprising: means for detecting whether transitions of an input datastream fall into a predetermined portion of a clock period of a clockutilized to sample the input data stream; and means for evaluatingwhether a phase-locked loop (PLL) has recovered a timing associated withthe input data stream according to occurrence of transitions in thepredetermined portion of the clock.
 17. The integrated circuit asrecited in claim 16 means for evaluating includes means for counting anumber of evaluation intervals that have one or more transitions thatfall into the predetermined portion of the clock period, generating acount thereof and determining if lock is achieved according to thecount.
 18. The integrated circuit as recited in claim 16, furthercomprising means for changing an output frequency of a variableoscillator circuit if it is determined that lock is not achieved.
 19. Amethod of acquiring a clock embedded in an input data stream, comprisingvarying an output of a variable oscillator until transitions of theinput data stream occurring in a predefined phase zone of a sample clocksampling the input data stream occur below an acceptable rate.
 20. Themethod as recited in claim 19 wherein a rate of transitions isdetermined according to a number of evaluation intervals having one ormore transitions occurring in the predefined phase zone.
 21. The methodas recited in claim 19 wherein the output of the variable oscillator isvaried by varying an impedance of the variable oscillator.
 22. Themethod as recited in claim 19 wherein varying the output of the variableoscillator comprises varying at least one of a control voltage and acontrol current supplied to the variable oscillator.
 23. An integratedcircuit for receiving an input data stream and locking to a clockembedded in the input data stream using a phase-locked loop, theintegrated circuit comprising: a phase zone detect circuit coupled todetermine if a transition of the input data stream occurs in apredetermined phase zone of a sample clock used to sample the input datastream; a counter circuit coupled to the phase zone detect circuit tosupply an indication of a number of evaluation intervals in which atleast one bit error occurs; a compare circuit coupled to compare theindication and a threshold value and to output a compare indication,thereby indicating if the phase-locked loop has locked to the input datastream; a variable oscillator circuit forming part of the phase-lockedloop; and a control circuit, responsive to the indication that lock isnot achieved, to vary the output of the variable oscillator circuit. 24.The integrated circuit as recited in claim 23 wherein: the phase zonedetect circuit includes a first data path and a second data path coupledto receive the input data stream, one of the first and second data pathsbeing delayed with respect to the other, thereby defining the phasezone, and wherein an output signal supplied from the first and seconddata paths are coupled to a logic circuit to be logically compared. 25.The integrated circuit as recited in claim 23 wherein the first datapath is a phase detector circuit coupled to provide an indication ofphase error between a recovered clock being used to sample the inputdata stream and the input data stream.
 26. The integrated circuit asrecited in claim 24 wherein the one of the first and second data pathsis delayed by delaying one of the clock and the data of the input datastream supplied to the one of the first and second data paths.
 27. Theintegrated circuit as recited in claim 23 wherein the second data pathincludes one or more selector circuits to select from a plurality ofclock frequencies.
 28. The integrated circuit as recited in claim 24wherein the first and second data paths are logically compared in anexclusive OR circuit.
 29. The integrated circuit as recited in claim 23further comprising: a variable impedance circuit forming part of thevariable oscillator circuit; and wherein the control circuit isresponsive to the indication that lock is not achieved, to vary thevariable impedance circuit to thereby adjust the output of the variableoscillator circuit.
 30. The integrated circuit as recited in claim 29wherein the control circuit adjusts the impedance by changing theimpedance to successively above and then below an initial value toprovide a gradually increasing swing around an initial impedance value.31. The integrated circuit as recited in claim 23 wherein thephase-locked loop is determined to be locked to the input data stream ifthe indication from the count circuit indicates that the number ofevaluation intervals in which at least one transition in a predeterminedphase zone occurs is below a predetermined threshold value.
 32. Theintegrated circuit as recited in claim 29, wherein the variableimpedance is a capacitance.
 33. The integrated circuit as recited inclaim 23 wherein the oscillator circuit is a tank circuit including aninductive element.
 34. The integrated circuit as recited in claim 23wherein the oscillator circuit is ring oscillator.
 35. The integratedcircuit as recited in claim 23 wherein the oscillator circuit is avoltage controlled oscillator (VCO).
 36. A method comprising determiningwhether a phase-locked loop (PLL) circuit that is coupled to an inputdata stream has locked to the input data stream according to whethertransitions of the input data stream occur below a predetermined rate ina predetermined portion of a sample clock period of a sample clockutilized to sample the input data stream.
 37. An apparatus comprising: avariable oscillator that is part of a phase-locked loop; and meanscoupled to the variable oscillator for determining whether thephase-locked loop has locked to an input data stream according towhether transitions of the input data stream occur in a predeterminedportion of a sample clock period at a rate that is below a predeterminedrate.
 38. A method comprising determining whether a phase-locked loop(PLL) circuit that is coupled to an input data stream has locked to theinput data stream according to whether transitions, occurring in apredetermined portion of a sample clock period of a sample clockutilized to sample the input data stream of the input data stream, occurbelow a predetermined rate.
 39. The method as recited in claim 38further comprising varying an output of a variable oscillator while thephase-locked loop is not locked to the input data stream.